Field of the Invention
This invention relates to preparation of silicon trichloride or silicon tetrachloride and to a gaseous mixture thereof. This invention is particularly directed to a process for preparing silicon trichloride and/or silicon tetrachloride by a process in which temperature control of the reactants is provided whereby to obtain desired product. This invention is particularly directed to a process for the preparation of silicon trichloride and to minimizing the amount of silicon tetrachloride produced as by-product in such synthesis. This invention is directed to a fluidized bed process for the preparation of silicon tetrachloride or silicon trichloride.
The hydrochlorination of elemental silicon takes place at temperatures above 260.degree. C, two reactions mainly taking place:
1. Hydrochlorination to trichlorosilane, and 2. Hydrochlorination to tetrachlorosilane, in accordance with the following equations: EQU 1. Si + 3 NCl .fwdarw. HSiCl.sub.3 + H.sub.2 - 56.4 kcal EQU 2. Si = 4 HCl .fwdarw. SiCl.sub.4 + 2 H.sub.2
in temperature ranges below about 400.degree. C it is principally Reaction 1 that takes place, i.e., it is mainly trichlorosilane that forms. At higher temperatures, Reaction 2 is increasingly favored, i.e., the tetrachlorosilane formation increases with the temperature.
As the temperature rises from 260.degree. toward 400.degree. C, the trichlorosilane content diminishes from approximately 90% at 260.degree. C to approximately 40% at 400.degree. C. As the temperature increases from 400.degree. to 500.degree. C, the trichlorosilane content further diminishes from 40% at 400.degree. C to about 10% at 500.degree. C. At temperatures above about 500.degree. C, the SiCl.sub.4 :HSiCl.sub.3 ratio remains constant at approximately 9:1.
Both of the reactions set forth above take place rapidly and are highly exothermic. The reaction system heats up locally and spontaneously to more than 1000.degree. C. As this occurs, the trichlorosilane content reduces to about 10% or less, while the tetrachlorosilane content amounts to 90% or more.
Unfortunately, of these two products the product of increasing technical and economic importance is the trichlorosilane. This is owing to the fact that trichlorosilane is useful in the semi-conductor field, for example, and is also useful as a basic substance in organosilane chemistry. Hence, it has become necessary to provide means to steer the hydrochlorination reaction of elemental silicon mainly towards the formation of trichlorosilane. Obviously, a prerequisite for this is to control the heat generated during the reaction so as to maintain a low reaction temperature, i.e., a temperature in the range of 260.degree. to 400.degree. C. If means are not provided to control this reaction, unfavorable low quantities of trichlorosilane are provided, this material being further converted in the process to tetrachlorosilane owing to the increasing reaction temperatures that prevail. It is not possible during this process to shift the temperature dependence to a higher temperature level by providing a higher hydrogen partial pressure.
Attempts have heretofore been made both in the solid bed and in the fluidized bed to commence the reaction at the lowest possible temperatures by introducing a catalyst, such as a copper catalyst, for example, and to retard the reaction such that lower temperatures are established and heat removal becomes possible. These efforts, however, have not as yet attained the desired success. The addition of metal catalysts does facilitate the reaction start-up at temperatures below 280.degree. C, but such expedient is not capable of preventing spontaneous heating up in the reaction bed to white-hot temperatures in excess of 500.degree. C. In the solid bed, either with or without a catalyst, an incandescent zone is immediately formed in which the reaction is directed toward tetrachlorosilane in accordance with equation 2 set forth above. The tetrachlorosilane is favored because the excess heat of reaction cannot be carried away rapidly enough. Even in the fluidized bed, with the rapid temperature control which is characteristic of this type of operation, the removal of heat is unsuccessful. Instead, the fluidized bed heats up locally on the bottom to a white heat so that even when the above-named conditions are produced in the fluidized bed, the tetrachlorosilane material predominates in the final product. In addition, these extreme process conditions necessitate great expense for repairs because they are extremely subject to trouble especially on account of the corrosion problems which occur employing hydrogen chloride at the elevated temperatures which are provided owing to the exothermic nature of the reaction and the inability to withdraw heat from the reaction zone.
Attempts have heretofore been made to remove the considerable local overheating that develops during this reaction, for example by feeding inert gases into the bed. For this purpose, experiments have been performed both with hydrogen and with nitrogen, as inert gases, in conjunction with gaseous tetrachlorosilane, the inert gas being mixed with the hydrogen chloride provided for the reaction and the hydrochlorination reaction being performed in a fluidized bed containing elemental silicon as the solid phase. In this manner, it was possible to produce the desired range of temperatures, but this mode of operation is disadvantageous in fluidized bed procedures especially because, due to the intense self-heating of the reaction, the inert gases have to be supplied in such large quantities. This results in an incomplete reaction of the hydrogen chloride since the inert gases in the fluidized bed tend to bubble and splash within the reaction zone and to effect intensive mixing as the reactants pass through the reaction zone during the established detention period. This high inert gas content coupled with the presence of large quantities of unreacted hydrogen chloride also results in considerable material losses owing to the removal of granular silicon solids from the fluidized bed. This, in turn, creates additional difficulties in the form of requiring a constant shifting of the lines through which the reactant and product streams pass and requiring cleaning of these lines and the reactor. The presence of such large quantities of hydrogen chloride tends to effect corrosion of the lines and reaction vessels employed to a substantially greater extent than would be provided if less unreacted hydrogen chloride were present in the product streams.
Lastly, the inert gas content carries away large amounts of trichlorosilane into the exhaust gas on account of its high partial pressure which is approximately 400 Torr at 15.degree. C and 30 Torr at -40.degree. C. This requires an expensive washing and distillation system for the recovery of the trichlorosilane, an obvious disadvantage.
It therefore became desirable to provide a process for the production of trichlorosilane by reaction of elemental silicon and HCl which was not characterized by local overheating of the reaction mixture, which could be carried out at a temperature of 260.degree. to 400.degree. C, which did not require the introduction of inert gas solids, which was not characterized by high quantities of unreacted hydrogen chloride in the product streams and which did not require expensive washing and distillation systems for the recovery of trichlorosilane.